The present invention relates to a soft, non-blocking polyurethane and, more particularly, to a soft, non-blocking thermoplastic polyurethane which is useful in medical and surgical devices.
A number of polymeric materials have been investigated for use in the fabrication of medical devices. One of these materials, silicone rubbers, although widely used, is disadvantageous because large amounts of reinforcing fillers and other additives such as plasticizers, catalysts, and the like are added to the polymer to achieve adequate physical properties such as tensile and tear strength.
As a result of several limitations, silicone rubber tubing is not suitable for use in the body for a prolonged period. Certain additives are susceptible to being extracted by body fluids. This is not only a potential source of contamination, but, absent the additive, there is a tendency for devices made from silicone rubbers to lose their shape and physical properties. The fillers used in silicone rubber are also a potential source of contamination and have been known to induce thrombosis. A further disadvantage of silicone rubber is that due to its limited physical-mechanical properties, a relatively large wall thickness is required in medical tubing and the like to achieve adequate strength and kink resistance. Silicone rubber cannot be used to fabricate finer diameter medical tubing having a narrow wall thickness.
Polyurethanes have arisen as a particularly desirable replacement for silicone rubber in certain applications. One disadvantage of polyurethane resins of the softness desired for many medical devices, e.g., resins having Shore A hardness less than 100, is surface blocking (tack) after extrusion or molding into desired shapes. To avoid this problem, many remedies have been developed in the art including the use of external mold release agents and the use of various antiblockers or detackifiers in admixture with the polymer. Most antiblocking agents/detackifiers are low molecular weight materials which have a tendency to migrate out or to leach (extract) out from the bulk/surface of the polymer. This represents a problem when polyurethanes when used as biomaterials (tubing, prostheses, implants, etc.). The presence of such low molecular weight extractables could effect their biocompatibility, hemocompatibility and surface degradation (fissuring, stress-cracking).
Although not related to biomaterials, U.S. Pat. No. 4,057,595 to Rauner et al. teaches the use of other compounds of detackifiers. This patent discloses a method for modifying the physical characteristics of polyurethane elastomers, particularly Spandex fibers, to reduce blocking, wherein the polyurethane contains, within the polymer chain, a siloxane-polyoxyalkylene block copolymer of the formula ##STR2## wherein x has an average value of 3 to 45, y has an average value of 8 to 198, Me is a methyl radical and R is an alkylene radical. These silicone modified polyurethanes contain an Si--O--C linkage which makes them less than completely satisfactory for certain medical applications. The Si--O--C linkage is susceptible to hydrolysis. Upon prolonged exposure to body fluids, these polyurethanes have a tendency to lose their physical characteristics.
Accordingly, there is a need in the art for relatively soft polyurethane resins that are non-tacky, non-toxic, and which are thermoplastic so that they can be readily fabricated into medical devices such as tubing, surgical dressings, intravenous solution bags, implant devices, and the like.